• The Singleton pattern "Ensure[s] a class has only one instance, and provide[s] a global point of access to it."
• The Singleton class is responsible for keeping track of its sole instance as follows:
  • it provides a static "uniqueInstance" property to store the instance
  • it provides a static "instance()" method that returns the instance
  • the instance() method also creates the unique instance on first invocation
• our Logger class from the Observer example is a Singleton

• relevant org.moock.logger.Logger source code:

// CONSTRUCTOR
org.moock.logger.Logger = function () {
  this.setLevel("INFO");   
}


// THE "uniqueInstance" STATIC PROPERTY
org.moock.logger.Logger.log = null;

// THE "instance()" STATIC METHOD
  /**
   * Returns a reference to the log instance.
   * If no log instance exists yet, creates one.
   */
  org.moock.logger.Logger.getLog = function () {
    // If no log exists...
    if (org.moock.logger.Logger.log == null) {
      // ...create one.
      org.moock.logger.Logger.log = new org.moock.logger.Logger();
    }

    // Return the log.
    return org.moock.logger.Logger.log;  
  }

• In ActionScript, there's no way to prevent another class from invoking the Logger constructor directly.
• In languages that can specify explicit access privileges (e.g., C++, Java), constructor invocation should be limited to Logger and its subclasses (i.e., "protected" in C++, or "default" or "protected" in Java)
• Singleton is a good alternative to a global variable because:
  • a global variable pollutes the global namespace (might conflict with other globals)
  • a global variable can't prevent multiple instantiations
• What if multiple applications are running, and each wants its own log?
• Make Logger subclasses, one for each application
• override getLog(), storing the Logger instance in the subclass's own static "log" property.